I really want to upgrade from S2 to S3 as I love tweaking the cars in any way I like (e.g. using the 'NewTweakOrder' mod on version 0.6H).

Three questions: (underlined below)

1. I feel completely clueless on what I'll actually be able to do with the 'new' vehicle mods / car editor feature. Is it just cosmetic modding (just the body shape with a selection of pre-determined engine / drivetrain options)?

2. For example, in S3 car editor, if I wish, can I adjust any car's power from say 10hp up to 10,000hp, or change transmission type from h-pattern to sequential, or change drivetrain from fwd to awd, or change rim size from 10" to 22" etc.?

3. More to the point; I'd like to be able to freely modify the cars' characteristics in rapid succession; but wondered even if I can, do you have to publicly publish these changes each time and get them approved before you can try them in-game?

Short answer: yes to 2) and no to 1) and 3). The editor allows you to create complete vehicles, as well as edit any existing one to your liking. There is also a "Test in LFS" button for local testing, and there is no obligation to publish mods.

As far as engine tweaks are concerned, I'm not sure 10k hp is possible, as values are based on physical parameters; you can definitely go over 1,000hp though. Also keep in mind that engine mass is calculated automatically, a more powerful engine will be heavier.

I would suggest going through some of the editor tutorials to see what and how you can tweak things, I believe you will find them in the vehicle mods forum.

In single player mode, you can do whatever you want with mods using the test mod feature.

If you want to use mods online, they must go through a review process before they are published. Subsequent updates to a mod are not actively reviewed, but significant changes to a mod after initial publication are not allowed - the mod must keep its original character.

There is documentation about mods in the LFS Manual: https://en.lfsmanual.net/wiki/Vehicle_Mods

There are links to LFS Editor guides and video tutorials, or how to submit your mod to be used online.

Thank you both, very helpful! I appreciate it

Query solved.

Quote from Bokujishin :

As far as engine tweaks are concerned, I'm not sure 10k hp is possible, as values are based on physical parameters; you can definitely go over 1,000hp though. Also keep in mind that engine mass is calculated automatically, a more powerful engine will be heavier.

More than 1,000 horsepower is definetely possible, highest I could get is 28,551hp from a 12 litre V16

Quote from Apex Cutter :

3. More to the point; I'd like to be able to freely modify the cars' characteristics in rapid succession; but wondered even if I can, do you have to publicly publish these changes each time and get them approved before you can try them in-game?

I think NTO, LFSTweak etc. could work with mods as they use the same car classes as default LFS cars do(not sure but atleast the naming is very similar), though there is no lfstweak for the current patch. You wouldnt get the detailed adjustments like demo cars would have but instead the cartype editor with power/torque multipliers, max gears, redline, weight modifier etc.

I would imagine it working online if its being run on a server with the modifier tag